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On September 30, 1999, a criticality accident occured at the Tokai nuclear fuel plant, Tokai-mura, Ibaraki Prefecture, Japan. The plant, operated by JCO Co. Ltd., a 100% subsidiary of Sumitomo Metal Mining Co. Ltd. , converts enriched uranium hexafluoride (UF6) to uranium dioxide (UO2) for use in nuclear fuel. Criticality accidents involve a self-sustaining chain reaction caused from handling of too large amounts of enriched uranium. The chain reaction continued for around 20 hours, before it could be stopped.

The uranium processed was enriched to 18.8% of uranium-235, rather than the 3-5% used for commercial light water reactor fuel. Material of this high enrichment grade was being produced for the experimental Joyo Fast Breeder reactor.
The plant has an annual production capacity of 715 tonnes of uranium for light water reactors, and 3 tonnes of uranium for fast breeder reactors.

The conversion process used in the Tokai plant is a wet chemical process:Schematic of process (STA)
The uranium oxide (U3O8) obtained in a first processing step (in the form of a powder) is dissolved in nitric acid (HNO3). The resulting uranyl nitrate solution or slurry (UO2(NO3)2) is pumped into a buffer tank. Then, the liquid is pumped into a precipitation tank, where ammonia (NH3) is added to precipitate ammonium diuranate (NH4)2U2O7 ("ADU"). The amount of uranium contained in this tank must be strictly controlled to avoid criticality. These controls however failed, and 16 kg instead of the permitted 2.4 kg were poured into the tank. About 6 kg were already sufficient to initiate a criticality under these circumstances.

In dry systems, criticality accidents often are quenched by various inherent effects, terminating the chain reaction automatically. In solution criticality accidents such as in Tokai however, the chain reaction may continue for some time after the first high peak and typically is followed by fissions of oscillating intensity, see the following graph:

The Tokai precipitation tank was equipped with a water-filled cooling jacket. After the chain reaction was initiated, this water possibly contributed to the effect that the reaction did not stop automatically after the first peak. Water is a good reflector and moderator to slow down neutrons for they can initiate further fissions of uranium-235 atoms. The chain reaction only stopped, after this water had been drained and boron (a neutron absorber) had been added.

Based on the results from analyzing the sample solution taken form the precipitation basin on October 20, 1999, the STA assumes that the entire nuclear fission number resulting from the criticality reaction is 2.5 · 1018, and the weight of uranium-235 that underwent fission was about 1 mg. Nearly half of the total fissions (1.2 · 1018) apparently occured during the first 25 minutes of the criticality. The total energy generated during the criticality event was estimated to have been about 22.5 kWh.

The chain reaction caused heavy releases of gamma and neutron radiation. Three workers were exposed to doses of up to 17 Sv (Sieverts), causing severe radiation sickness. The worker exposed to the higest dose died on December 21, 1999. The worker exposed to the second highest dose of 6 - 10 Sv died on April 27, 2000. 68 other persons were irradiated at lower levels. Among them were the workers who stopped the chain reaction: they were exposed to doses of up to 119.79 mSv, exceeding the 100 mSv limit for emergency situations. The annual dose limit for workers is 50 mSv (while ICRP currently recommends 20 mSv). As of October 7, 1999, radiation levels remained high inside the plant building, preventing inspection of the damage inside the plant.
First reports that an explosion had blown a hole into the roof of the facility have not been confirmed (see STA Oct 7, 1999 ).

On November 4, 1999, the Science and Technology Agency submitted a report to the Nuclear Safety Commission, containing estimates of radiation exposures during the criticality accident, based on an analysis of uranium solution sampled from a tank inside the fuel processing plant and neutron levels monitored around the plant. The report says that

the radiation dose received at a distance of 80 meters from the accident site (that is the nearest boundary of the plant) was estimated at 75 mSv* for the first 25 minutes of the criticality accident, and 160 mSv for the whole criticality period of 20 hours;

the radiation dose received at a distance of 350 meters from the plant (that was the evacuation boundary), was estimated at almost equal to the annual permissible dose of 1 mSv for the first 25 minutes, and 2 mSv for the whole event.

* this figure was revised to 30 mSv, according to Yomiuri Shimbun of Dec. 12, 1999. Details are not known yet.

Since the evacuation started only 5 hours after the begin of the criticality, residents may have received doses of more than 75 mSv, that is 75 times the permissible annual dose of 1 mSv.
The government has so far said 69 people were exposed to radiation, but the latest survey says the number of affected people could increase.

On January 31, 2000, the Science and Technology Agency reported that the JCO Co. nuclear accident exposed 119 people to levels of radiation that were higher than the maximum dose of one millisievert, considered safe for an ordinary person to be exposed to in a period of one year. The agency checked radiation doses received by 207 people living within a radius of 350 meters from the accident site, 148 JCO employees who were on its premises at the time of the accident, 60 firefighters and government officials who attended the scene, and 24 JCO technicians who entered the plant in an attempt to halt the self-sustaining nuclear reaction of a uranium solution. Among local residents, the highest radiation dose recorded was 21 millisieverts. The recipient lived right next to the plant and took a long time to evacuate, the agency said. (Yomiuri Feb. 1, 2000)
The estimate of the total number of persons exposed to radiation from the accident was raised to 439. Of those affected, 207 were local people who lived and
worked within a 350 m radius of the plant. (Reuters Jan 31, 2000)

On Oct. 14, 2000, the Science and Technology Agency raised the number of people exposed to radiation to 667. The number includes 229 people - passers-by, government officials and those from new organizations - who have newly been found to have been exposed to radiation in the accident. Of the 229, 56 were overexposed, registering more than 1 millisievert of radiation. One person registered 7.2 millisieverts. (Mainichi Oct. 15, 2000)

Contamination monitoring of the JCO Facility work site was conducted on the
afternoon of Oct. 18, 1999.
> View survey map (STA)

Dark hexagon: settling tank (gamma dose rate 55 mSv/h)

Circle: gamma radiation dose rate in air

Triangle: swipe sample from surface

Rectangle with diagonal line: "thermal" or "warm" scrubber (???)

During the accident, monitoring of neutron and gamma dose rates started only several hours after the begin of the chain reaction. Since recent analysis (see above) shows that about half of the fissions occured during the first 25 minutes, the significance of such delayed monitoring is very limited: At the boundary of the facility, gamma dose rates of 0.84 mSv/h and neutron dose rates of 4.5 mSv/h were recorded, while the dose limit for members of the public is 1 mSv per year.
Two minutes after the accident, neutron radiation levels of 0.26 micro-Sv/h, that is 10 to 100 times the normal level, were detected at a laboratory of the Japan Atomic Energy Research Institute in Nakamachi, located about two kilometers from the JCO plant.

While gamma radiation is hazardous only while it is being emitted, neutron radiation produces new radioactive isotopes in irradiated matter - it activates the matter. These newly formed isotopes, such as sodium-24 and zinc-65, continue to emit radiation at their respective half-lives, even after the neutron radiation has stopped. On the one hand, this effect is a matter of concern for the residents living nearby the plant, on the other hand it can be used to estimate the neutron dose rates received at such places, where no neutron monitoring had been performed.

Personal radiological data calculated from whole-body counting of Na-24 is available from STA:

calculated dose (neutron plus gamma) in mGy, based on a coefficient of 0.5 - 3 µGy per Bq of Na-24 (according to IAEA Technical Report Series No.211). It is not clear how to convert these combined values to mSv.

The chain reaction also produced gazeous fission products, such as radioactive isotopes of iodine, krypton, and xenon. These, as well as radioactive cesium, probably have been released into the environment.
The following is an inventory of fission products detected within 3 km of the
accident site. The figures were taken from newspapers and TV reports, and have been compiled by CNIC:

On Oct 11, 2000, six former executives and current employees of JCO Co. were arrested on suspicion of negligence resulting in two employees' deaths. Ibaraki Prefectural Police spent more than one year investigating the nuclear accident and have apparently concluded that JCO's systematic rule violations and a lack of safety measures jointly caused the disaster. Ibaraki police arrested Kenzo Koshijima, 54, who was the head of the JCO Tokai plant where the accident took place on Sept. 30, and five other officials of the company. (Mainichi Shimbun Oct. 12, 2000)

Six former top officials at JCO Co. pleaded guilty to charges of negligence resulting in death as the trial opened into Japan's worst nuclear accident. The pleas were entered at the initial hearing at the Mito District Court in Ibaraki Also, current JCO Co. president Tomoyuki Inami admitted to a charge that the company violated the nation's nuclear regulations law, said Mito District Court spokesman Michiru Sakurai. (Reuters Apr 23, 2001)

Three residents of Tokai, Ibaraki Prefecture, who were exposed to radiation during a nuclear fuel plant accident in 1999, are set to sue the plant's operator and its parent company for damages. The three will file a suit in early September 2002 with the Mito District Court demanding that JCO Co. and its parent company, Sumitomo Metal Mining Co. pay them compensation for damages to their health and mental anguish caused by the accident. They have not yet decided on the amount of damages they will demand from JCO and Sumitomo.
(Mainichi Shimbun August 19, 2002)

Six executives who admitted negligence in the 1999 incident received suspended prison terms at a court on March 3, 2003.
The former head of the plant, Kenzo Koshijima, 56, received a five-year prison term suspended for three years and was fined 500,000 yen (US$ 4,231).
The other five executives were given suspended prison terms of up to three years.
(Reuters March 3, 2003)

On April 18, 2003, Sumitomo Metal Mining Co. announced its fuel-processing subsidiary, JCO Co., in Tokaimura, Ibaraki Prefecture, the site of the nation's worst nuclear accident in September 1999, would abandon uranium reprocessing operations. JCO would not, however, be dissolved and would continue to compensate victims of the accident and work on the maintenance of low-level radioactive waste still remaining in the facility, the company said. (Yomiuri Shimbun April 19, 2003)

A panel of nuclear experts investigating Japan's worst nuclear accident in Tokaimura, Ibaraki Prefecture, in 1999 presented its final report on Sep. 17, 2004, at a meeting of the Atomic Energy Society of Japan held in the city of Kyoto. The panel, led by Hideki Nariai, a professor emeritus at Tsukuba University, criticized the government's nuclear safety policy leading to the accident and asked the government for improvements.

Meanwhile, JCO is planning to demolish the Tokai plant, despite calls for preserving it as evidence to pass down memories of the tragedy. The company said, it has already settled some 7,000 damage claims out of court by paying a total of 14.86 billion yen (US$ 136 million). (Kyodo Sep. 17, 2004)
On May 11, 2005, JCO Co said it has obtained approval from the government to dismantle its nuclear fuel-processing facility at Tokaimura, Ibaraki Prefecture, where a fatal accident occurred in 1999. The company will start dismantling operations in June 2005 and hopes to complete them in about 10 months. The facility could be rebuilt in the future.
(Japan Times May 12, 2005)
On June 6, 2005, JCO Co. began dismantling the facility. The company will complete the operation in March 2006, JCO said, adding that some of the dismantled facilities will be contained in drums and stored at the site so they can be rebuilt in the future. (Japan Times June 6, 2005)

JCO Criticality Accident and Local Residents:
Damages, Symptoms and Changing Attitudes, Data and Analysis of the Results of a Field Survey of Tokai-mura and Naka-machi Residents
By Koichi Hasegawa and Yuko Takubo, Citizens' Nuclear Information Center, Tokyo, June 2001, 54 p.
> Download full text

Village government of Tokai:Memories to be Handed Down , Tokai, 184 p., 2010 (in Japanese)[Seeking to hand down lessons from Japan's first criticality accident, the village of Tokai, Ibaraki Prefecture, has published an anthology of essays contributed by people involved in the Sept. 30, 1999, fatal disaster.
Around 90 contributors, including nuclear experts, local officials as well as residents of the Tokai area, share their experiences in facing the unprecedented accident and reflect on how they could have done better.]

A total of about 60 criticality accidents has been reported worldwide. Most of them occured in the 1940's to 1970's in nuclear (weapons) research laboratories.

"In the United States there has been only one incident in which criticality was reached at a commercial nuclear fuel processing plant. That occurred in 1964 at a plant, no longer in operation, near Charlestown, Rhode Island.
A worker accidentally set off the reaction when he poured liquid uranium into a tank and died after being exposed to 1,000 times the lethal dose of radiation." (San Jose Mercury News, Oct. 1, 1999)

A near-criticality incident occured on May 28/29, 1991, at General Electric's Wilmington (North Carolina) nuclear fuel fabrication facility. Approximately 150 kilograms of uranium were inadvertently transferred from safe process tanks to an unsafe tank located at the waste treatment facility, thus creating the potential for a localized criticality safety problem. The excess uranium was ultimately safely recovered when the tank contents were centrifuged to remove the uranium-bearing material.
(for details, see: Potential Criticality Accident at the General Electric Nuclear Fuel and Component Manufacturing Facility, May 29, 1991, Investigation Report, Nuclear Regulatory Commission , Washington, DC., NUREG-1450, August 1991, 227 p.)